A Battery Energy Storage System (BESS) is an indispensable part of the Direct Current (DC) grid. For example, in the case of a High-Voltage Direct Current (HVDC) off shore wind farm, the wind energy penetrations are very high and the wind power fluctuations may affect the stability of the system. In this situation, a BESS can be incorporated into the system to smooth the intermittency and the short term wind power fluctuations. In Medium-Voltage Direct Current (MVDC) systems, normally there are a wide variety of loads which include high power propulsion loads and pulsed loads. These loads are connected to the MVDC bus through power converters, which are known to introduce a great deal of pulsed current and ripple current into the MVDC system. These two kinds of current exhibit a wide frequency range, which may lead to voltage oscillations on the MVDC bus. A BESS incorporating active filter functions is a good choice to overcome this power-quality issue. As low voltage fault and grounding fault issues commonly exist in the DC grid, low voltage fault ride through functions and current limiting functions are also needed in a BESS for DC grid applications.
As illustrated with reference to FIG. 1, a traditional BESS is mainly focused on the Alternating Grid (AC) grid application. In the traditional system, an energy storage battery bank is connected to the AC grid through a bidirectional DC-AC converter. As a result of the low battery cell voltage (e.g., LiFePO battery's cell voltage is 3.2V), in order to build a high voltage battery bank, hundreds of battery cells need to be connected in series in the battery bank. In order to ensure safe operation of this battery bank system, the state of charge (SOC) of each battery cell in the same series battery branch must be balanced with each other, utilizing a battery management system (BMS). The more battery cells that are connected in a series battery branch, the higher the complexity, the higher the failure rate and greater the cost of the BMS. As a consequence, the overall energy storage battery bank voltage is typically lower than 1000V. In a medium or high voltage rated AC grid utilization, the line frequency transformer works as an interface between the BESS and the AC grid. Unfortunately, line frequency transformers are not suitable for DC grid applications.
While the Battery Energy Storage System (BESS) is an indispensable component of the DC grid, the power rating and voltage rating of battery energy storage converters currently known in the art are not suitable for Medium Voltage Direct Current (MVDC) or High Voltage Direct Current (HVDC) applications.
Accordingly, what is needed in the art is a Battery Energy Storage System (BESS) that is compatible with Medium Voltage Direct Current (MVDC) or High Voltage Direct Current (HVDC) applications.